The present invention relates to a flexible, boil resistant film laminate and to packages, for example, retortable food pouches fabricated therefrom.
Certain packaging applications, for example, retort packages, require that the packaging material constitute a good barrier to the passage of oxygen and moisture vapor. In addition, the packaging of certain foods require that a minimum of oxygen or air are transmitted to the contents. Cheese is one food product which should be packaged to the exclusion of oxygen.
Oriented polyacrylonitrile (PAN) film has excellent oxygen barrier properties. Its moisture barrier properties, however, are less than desirable for certain packaging purposes. Furthermore, it is not sealable to itself. Both deficiencies can be overcome by applying to one surface of the PAN a layer of a thermoplastic material having good moisture barrier properties. For some purposes it is desirable to bond the PAN film to itself. It is necessary, therefore, to consider the imposition of an adhesive or bonding system between either two layers of PAN or a layer of PAN and a thermoplastic moisture barrier film. By providing an effective adhesive bond between such films the result will be satisfying a need that has developed in the food packaging industry. In recent years, the industry has shown intense interest in the concept of pouch-packaged foods which, among other advantages, do not require freezing for their preservation and can therefore dispense with costly and energy intensive refrigerated transportation and storage facilities. Much effort has gone into the development of a flexible food pouch which not only can withstand the rigors of sterilization and later reheating, e.g., microwave heating, and provide various properties which are sufficient to adequately protect the contents during storage, but which also employ materials which are toxicologically safe.
In accordance with U.S. Pat. No. 3,453,173, a polyolefin-polyacrylonitrile laminate, which is said to possess superior heat-sealed strength and excellent barrier properties to the transmission of gases and is adaptable for the construction of food containers, is prepared by bringing the polyolefin surface and the polyacrylonitrile surface together and adhering them to each other through an adhesive. Either or both surfaces can be pre-treated in some manner in an effort to make them more adherent, e.g., by treatment with a gaseous mixture of boron trifluoride, as described in British Pat. No. 834,196; by flame treatment or by treatment by corona discharge, as described in U.S. Pat. No. 2,632,921. Among the adhesives employed in the manufacture of the laminate is an ethylene vinylacetate copolymer described in U.S. Pat. No. 2,200,429. Because the conditions of preparation and the adhesive contemplated are not conducive to the formation of primary valence bonding, the interlaminar adhesion in boiling water would not be expected to be strong. Resort also has been had to metalizing the PAN film in order to employ the metal layer as an effective surface through which adhesion can be accomplished to the water vapor barrier film.
It is an object of the present invention to present polyacrylonitrile film laminates or composites of two or more PAN layers bonded together with a unique and effective bonding system.
It is an object of the present invention to present polyacrylonitrile film laminates or composites of PAN and a thermoplastic moisture barrier layer bonded together with a unique and effective bonding system.
It is a further object to present a food or product pouch or tube made from said laminate.
It is yet another object of the invention to present a method for improving the bond strength between films of the type described above.